Computerized tomography (CT) involves the imaging of the internal structure of an object by collecting several projection images (“radiographic projections”) in a single scan operation (“scan”), and is widely used in the medical field to view the internal structure of selected portions of the human body. Typically, several two-dimensional projections are made of the object, and a three-dimensional representation of the object is constructed from the projections using various tomographic reconstruction methods. From the three-dimensional image, conventional CT slices through the object can be generated. The two-dimensional projections are typically created by transmitting radiation from a “point source” through the object, which will absorb some of the radiation based on its size, density, and atomic composition, and collecting the non-absorbed radiation onto a two-dimensional imaging device, or imager, which comprises an array of pixel detectors (simply called “pixels”). Such a system is shown in FIG. 1. Typically, the point source and the center of the two-dimensional imager lie on a common axis, which may be called the projection axis. The source's radiation emanates toward the imaging device in a volume of space defined by a right-circular, elliptical, or rectangular cone having its vertex at the point source and its base at the imaging device. For this reason, the radiation is often called cone-beam (CB) radiation. Generally, when no object is present within the cone, the distribution of radiation is substantially uniform on any circular area on the imager that is centered about the projection axis, and that is within the cone. However, the distribution of the radiation may be slightly non-uniform, while having rotational symmetry about the projection axis. In any event, any non-uniformity in the distribution can be measured in a calibration step and accounted for. The projection axis may not be at the center of the imager or the center of the object. It may pass through them at arbitrary locations including very near the edge.
In an ideal imaging system, rays of radiation travel along respective straight-line transmission paths from the source, through the object, and then to respective pixel detectors without generating scattered rays. However, in real systems, when a quantum of radiation is absorbed by a portion of the object, one or more scattered rays are often generated that deviate from the transmission path of the incident radiation. These scattered rays are often received by “surrounding” pixel detectors that are not located on the transmission path that the initial quantum of radiation was transmitted on, thereby creating errors in the electrical signals of the surrounding pixel detectors. Furthermore, in typical two-dimensional imagers, the radiation meant to be received by a pixel is often distributed by various components of the imager (e.g., scintillation plate), and received by surrounding pixels. Also, there is typically some electrical cross-talk in the electrical signals of the pixel detectors caused by the electrical circuitry that reads the array of pixel detectors. These two effects are often characterized by a point-spread function (PSF), which is a two-dimensional mapping of the amount of error caused in surrounding pixels by a given amount of radiation received by a central pixel. The surface of the PSF is similar to the flared shape of a trumpet output, with the greatest amount of error occurring in pixels adjacent to the central pixel. Each of these non-ideal effects creates spatial errors in the pixel data generated by the two-dimensional imager.
The scattered radiation causes artifacts (e.g., phantom images) and loss of resolution and contrast in the CT image slices produced by the radiation imaging system. The scattered radiation can also cause numerical errors in the image reconstruction algorithms (generally referred to as “CT number problems” in the art). All of the foregoing lead to image degradation. Accordingly, there is a need in the computerized tomography field to reduce the impacts of these spatial and temporal errors.